1. Field of the Invention
The present invention relates to devices and methods for measuring a state of polarization as well as the spectral content of each picture element (pixel) of a target scene and, more particularly, to devices and methods for measuring a state of polarization as well as the spectral content of each pixel in a target scene in real time.
2. Related Art
Identifying the state of polarization of an electromagnetic wave by determining the Stokes polarization vector components of the wave is known. In particular, an electromagnetic wave, such as a spectral band of light, or of electromagnetic radiation in any spectral band, may be characterized as having four Stokes vector components (s0, s1, s2, and s3). The component s0 is proportional to the intensity of the wave. The components s1, s2, and s3 may be related to the orientation of the polarization, e.g., an ellipse and its ellipticity.
One way of measuring the Stokes vector components (s0, s1, s2, and s3) is to place two polarizers and a retarder in the optical path sequentially. Insertion of a first polarizer into an optical path gives a measure of one of the linear polarizations and a second polarizer is also inserted to give the other linear polarization. A retarder is further inserted into the optical path to retard a signal having a given sense of polarization in phase relative to a signal having another sense, where the two senses are generally orthogonal to each other. Output from the retarder is a signal containing data that can be used to calculate the phase when the linear components are known. The disadvantage of this approach is that it involves moving parts, since these optical components must be placed successively in the optical path. Also, in a dynamic scene, a polarimeter using moving parts would give smeared results, since the scene could change during the times that the polarizers are being changed.
Additionally, measurement of the spectrum of each pixel in a scene is currently accomplished by using some type of scanning spectrometer such as a grating spectrometer or a Michelson interferometer. The grating spectrometer collects light from the scene and scans it across a detector. The grating is designed so that there is a near one-to-one correspondence between the wavelength of radiation incident on the detector and the scan angle of the grating. The Michelson interferometer uses a scanning mirror to collect an interferogram, the Fourier transform of which yields the spectrum of the scene. The disadvantage of both of these means of generating the spectral content of each pixel is that they both require moving parts, namely a scanning grating or a scanning mirror, to collect spectral information for each pixel in a target scene.
In order to avoid moving polarizers into and out of the beam, a system for spectropolarimetry was described by Kazuhiko Oka and Takayuki Kato in “Spectroscopic Polarimetry with a Channeled Spectrum”, published in Optics Letters, Vol. 24, No. 21, Nov. 1, 1999. In particular, Oka and Kato employ a pair of birefringent retarders and an analyzer to modulate light so that the state of polarization of the light varies with frequency. The modulated light is then passed to a grating spectrometer or spectrum analyzer and then to a computer where, through Fourier analysis, the state of polarization of the modulated light is determined. A disadvantage of this approach is, again, the necessity for moving parts, namely a scanned diffraction grating, in the spectrometer. Also, Sabatke, et al., in Optical Engineering Vol. 41, No. 5, May 2002, describe an imaging spectropolarimeter that uses two optical retarders and a polarizer, together with a computed tomographic imaging spectrometer (CTIS), to measure a complete Stokes vector while employing no moving parts. However, the Sabatke system suffers from a deficiency of a limited spatial resolution by the need to use most of the focal plane area for higher grating orders.